1. Field of Invention
The present invention relates generally to a device and method forming stacks of flat elements, and in particular to a device and method for forming stacks of sheets such as sheets of paper in a stacking region.
2. Description of the Related Art
Various methods and devices are conventionally used for stacking flat elements such as sheets of paper. The term “flat elements” refers, in particular, to individual sheets of paper, film, plastics material or the like having a two-dimensional shape. The terms “stack” and “partial stack” refer to accumulations of flat elements located one above another. The term “stacking region” refers to the place or region at which the (partial) stack is formed from the conveyed flat elements.
In the paper-processing industry, what are known as collecting stations are used to convey individual sheets, formed for example using a cutting means by cutting from a running web, continuously, i.e., without interruption, to a stacking region from which they are positioned one above another to form stacks. During this accumulation of sheets in the stacking region, the stacks having a defined predetermined number of sheets generally have to be conveyed away from the stacking region for further processing. However, to avoid disrupting the operation of the machine as a whole, sheets continue to be fed into the stacking region without interruption. Thus, during the transfer of a finished stack from the main stack carrier, an auxiliary stack carrier temporarily takes over the further stacking of the sheets in the stacking region until the main stack carrier is emptied and can once again take over the stacking of the sheets, at which point the partial stack formed in the auxiliary stack carrier is transferred to the main stack carrier. To accomplish this task, the auxiliary stack carrier is brought into the stacking region when the main stack carrier fills up and is removed when the main stack carrier returns.
The transfer of the partial stack from the auxiliary stack carrier withdrawing from the stacking region to the main stack carrier is critical. For as the auxiliary stack carrier has a specific thickness, a wave is formed in the lower portion of the partial stack at the moment at which the partial stack leaves the auxiliary stack carrier and is deposited on the main stack carrier. The formation of a wave of this type causes the lower region of the partial stack to be deposited misaligned and offset relative to the remaining portion of the partial stack located thereabove. This adverse effect can be further exacerbated by the friction produced between the upper side of the auxiliary stack carrier and the underside of the partial stack, as a result of which the lower layers of the partial stack are entrained during the withdrawal movement of the auxiliary stack carrier from the stacking region. If merely the main stack carrier performs a compensatory stroke movement in the vertical direction in order to compensate for the thickness of the auxiliary stack carrier, which has already been removed at that point from the stacking region, the offset of the lower region or the lower layers of the partial stack formed by the wave continues up to the end in the direction of the withdrawal movement of the auxiliary stack carrier. This creates a shoulder in the stack known as an S-bend, which often constitutes a quality defect. Particularly in the paper-processing industry, it is usually necessary to produce substantially straight stack edges in order not to impede the subsequent processing of the sheets, which is especially important in high-grade papers.
In an attempt to solve this problem, EP 1 262 435 A1 proposes a method and a device in which a second auxiliary stack carrier is provided in addition to a first auxiliary stack carrier. The second auxiliary stack carrier is arranged on the opposing side of the stacking region in relation to the first auxiliary stack carrier. Once the first auxiliary stack carrier has been introduced and the second auxiliary stack carrier has reached a position opposite the first auxiliary stack carrier, the second auxiliary stack carrier is moved, synchronously with the first auxiliary stack carrier, into a central position in the stacking region from which it is withdrawn from the stacking region in the opposite direction of the removal of the first auxiliary stack carrier. The partial stack formed on the second auxiliary stack carrier is then deposited on a pallet positioned on the main stack carrier located below the plane formed by the two auxiliary stack carriers. Although a respective wave is formed at the mutually facing ends of the two auxiliary stack carriers, these two waves are oriented away from each other and thus compensate for one another. Thus, the synchronous symmetrical removal of the two auxiliary stack carriers leads to depositing the partial stacks substantially without edge misalignment. However, in such a device, the arrangement of the second auxiliary stack carrier necessitates an expensive construction and a complex control means, which in turn leads to higher production, operating and maintenance costs.
EP 0 896 945 B1 proposes the use of a plurality of alignment strips and/or plates for the active rectangular alignment of the pallet and the stack of sheets located thereon in order in this way to compensate for deformation or edge misalignment in the stack. However, this known device is unsuitable for heavyweight stacks and large-format sheets. What is needed is a solution that is simple in terms of construction and control in comparison to the conventional devices discussed above, while at the same time allowing stacks to be exchanged continuously and without impairing the quality of the stacks.